---
title: What is BAML?
---

The best way to understand BAML and its developer experience is to see it live in a demo (see below).
 

### Demo video
Here we write a BAML function definition, and then call it from a Python script.

<iframe src="https://www.youtube.com/embed/gxckvkNg6KM?si=8Zj8x_tsvZES8asd" title="BAML Demo Video" allow="autoplay; fullscreen" allowtransparency="true" frameborder="0" scrolling="no"  msallowfullscreen width="640" height="352"></iframe>

### Examples
- [Interactive NextJS app with streaming](https://baml-examples.vercel.app/examples/stream-object)
- [Starter boilerplates for Python, Typescript, Ruby, etc.](https://github.com/boundaryml/baml-examples)

### High-level Developer Flow

<Steps>
### Write a BAML function definition
```baml main.baml
class WeatherAPI {
  city string @description("the user's city")
  timeOfDay string @description("As an ISO8601 timestamp")
}

function UseTool(user_message: string) -> WeatherAPI {
  client "openai-responses/gpt-5-mini"
  prompt #"
    Extract.... {# we will explain the rest in the guides #}
  "#
}
```
Here you can run tests in the VSCode Playground.

### Generate `baml_client` from those .baml files.
This is auto-generated code with all boilerplate to call the LLM endpoint, parse the output, fix broken JSON, and handle errors.
<img src="/assets/vscode/baml-client.png" />

### Call your function in any language
with type-safety, autocomplete, retry-logic, robust JSON parsing, etc..
<CodeGroup>
```python Python
import asyncio
from baml_client import b
from baml_client.types import WeatherAPI

def main():
    weather_info = b.UseTool("What's the weather like in San Francisco?")
    print(weather_info)
    assert isinstance(weather_info, WeatherAPI)
    print(f"City: {weather_info.city}")
    print(f"Time of Day: {weather_info.timeOfDay}")

if __name__ == '__main__':
    main()
```

```typescript TypeScript
import { b } from './baml_client'
import { WeatherAPI } from './baml_client/types'
import assert from 'assert'

const main = async () => {
  const weatherInfo = await b.UseTool("What's the weather like in San Francisco?")
  console.log(weatherInfo)
  assert(weatherInfo instanceof WeatherAPI)
  console.log(`City: ${weatherInfo.city}`)
  console.log(`Time of Day: ${weatherInfo.timeOfDay}`)
}
```

```go Go
package main

import (
    "context"
    "fmt"
    
    b "example.com/myproject/baml_client"
    "example.com/myproject/baml_client/types"
)

func main() {
    ctx := context.Background()
    
    weatherInfo, err := b.UseTool(ctx, "What's the weather like in San Francisco?")
    if err != nil {
        panic(err)
    }
    
    fmt.Printf("%+v\n", weatherInfo)
    fmt.Printf("City: %s\n", weatherInfo.City)
    fmt.Printf("Time of Day: %s\n", weatherInfo.TimeOfDay)
}
```

```ruby Ruby
require_relative "baml_client/client"

$b = Baml.Client

def main
  weather_info = $b.UseTool(user_message: "What's the weather like in San Francisco?")
  puts weather_info
  raise unless weather_info.is_a?(Baml::Types::WeatherAPI)
  puts "City: #{weather_info.city}"
  puts "Time of Day: #{weather_info.timeOfDay}"
end
```

```python Other Languages
# read the installation guide for other languages!
```
</CodeGroup>
</Steps>

Continue on to the [Installation Guides](/guide/installation-language) for your language to setup BAML in a few minutes!

You don't need to migrate 100% of your LLM code to BAML in one go! It works along-side any existing LLM framework.